Iteratively merges into local branch a list of pull requests in sequence using the GitHub and Git CLIs

github-merge-prs-into-branch.sh

#!/bin/bash

set -e

# Name of the branch to merge all the pull requests into.
MERGE_BRANCH="staging"

# Name of the branch to hard reset to before merging others into it.
RESET_BRANCH="master"

# Multi-line string of open, non-draft pull requests to merge locally.
# Each line is a space-delimited string of "pr_number base_branch head_branch".
PULL_REQUESTS=$(
  gh pr list \
    --state open \
    --draft=false \
    --json number,baseRefName,headRefName \
    -q '.[] | "\(.number) \(.baseRefName) \(.headRefName)"'
)

# Map each line of the pull requests query result into a simple array
# so that we can more easily iterate the data later.
PR_INFO=()
while read -r pr_number base_branch head_branch; do
  PR_INFO+=("$pr_number $base_branch $head_branch")
done <<< "$PULL_REQUESTS"

# Simple array of pull request numbers in the order they should be merged.
ORDERED_PRS=()

# ----------------------------------------------------------------------------

# $1 = message to log
function log_info() {
  echo "" >&2
  echo -e "> ${1}" >&2
  echo "" >&2
}

# Fetches all history.
function git_fetch() {
  git fetch --all --force --prune --prune-tags --no-tags --quiet
}

# Checks out a pull request into the current git repo.
# $1 = branch to checkout and merge pull requests into (required)
# $2 = remote branch to hard reset to (defaults to $1)
function git_checkout() {
  local branch="${1}"
  local reset_to="${2:-$1}"

  log_info "checking out branch '${branch}'"

  git checkout -B ${branch} origin/${branch}
  git reset --hard origin/${reset_to}
}

# Get the head branch name of the currently checked out git repository.
# $1 = pull request url or number (optional)
function get_head_ref() {
  local pr_id="${1}"

  gh pr view ${pr_id} --json headRefName -q '.headRefName'
}

# $1 = pull request url or number (optional)
function merge_head_ref() {
  local pr_id="${1}"
  local head_ref="$(get_head_ref ${pr_id})"

  log_info "merging branch '${head_ref}'"

  git merge "origin/${head_ref}" --no-edit --no-ff --no-verify
}

# Iterates the pull requests to identify the dependency order for merging.
# Outputs the pull request numbers to new array variable `ORDERED_PRS`.
function calculate_merge_order() {
  local branch="${1}"

  for pr_info in "${PR_INFO[@]}"; do
    read -r pr_number base_branch head_branch <<< "${pr_info}"
    if [[ "${base_branch}" == "${branch}" ]]; then
      ORDERED_PRS+=("${pr_number}")
      calculate_merge_order "${head_branch}"
    fi
  done
}

# Iterates the pull requests in their dependency order and merges them locally.
function merge_ordered_prs() {
  for pr_number in "${ORDERED_PRS[@]}"; do
    head_ref=$(get_head_ref ${pr_number})
    merge_result=$(merge_head_ref ${head_ref})
    has_conflicts=$(git status | grep --quiet "fix conflicts" && echo 1 || echo 0)

    if [[ "${has_conflicts}" == "1" ]]; then
      log_info "merge conflicts, aborting"
      git status
      exit 1
    fi
  done
}

# ----------------------------------------------------------------------------

git_fetch

git_checkout "${MERGE_BRANCH}" "${RESET_BRANCH}"

calculate_merge_order "${RESET_BRANCH}"

merge_ordered_prs